Reappraisal of the phosphorus requirement of lactating dairy cows

Abstract

Our aim was to evaluate the effects of a low or high dietary phosphorus (P) concentration during the dry period, followed by either a high or low dietary P concentration during the first 8 weeks of lactation, on plasma Ca concentrations, feed intake and lactational performance of dairy cattle. Sixty pregnant multiparous Holstein Friesian dairy cows were assigned to a randomized block design with repeated measurements and dietary treatments arranged in a 2 × 2 factorial fashion. The experimental diets contained 3.6 (Dry-HP) or 2.2 (Dry-LP) g P/kg dry matter (DM) during the dry period, and 3.8 (Lac-HP) or 2.9 (Lac-LP) g P/kg DM during 56 days after calving period. In dry cows, plasma Ca concentrations were 3.3% greater when cows were fed 2.2 instead of 3.6 g P/kg DM. The proportion of cows being hypocalcemic (plasma Ca concentrations < 2 mM) in the first week after calving was lowest with the low P diets both during the dry period and lactation. Plasma Ca concentrations in week1 to 8 after calving were affected by dietary P level in the dry period and in the lactation period, but no interaction between both was present. Feeding Dry-LP instead of Dry-HP diets resulted in 4.1% greater plasma Ca values, and feeding Lac-LP instead of Lac-HP diets resulted in 4.0% greater plasma Ca values. After calving, plasma Pi concentrations were affected by a 3-way interaction between sampling day after calving, and dietary P levels during the dry period and lactation. From d 1 to d 7 post-partum, cows fed Lac-HP had increased plasma Pi concentrations, and the rate appeared to be greater in cows fed Dry-LP vs. Dry-HP. In contrast, plasma Pi concentrations decreased from d 1 to d 7 post-partum in cows fed Lac-LP, and this decrease was at a higher rate for cows fed Dry-HP vs. Dry-LP. After d 7, plasma Pi concentrations remained rather constant at 1.5-1.6 mM when cows received Lac-HP while with Lac-LP plasma Pi concentrations reached stable levels (i.e. 1.3-1.4 mM) at d 28 after calving. Milk production, DM intake, and milk concentrations of P, Ca, fat, protein and lactose were not affected by any interaction nor the levels of dietary P. It is concluded that the feeding of diets containing 2.2 g P/kg DM during the last 6 weeks of the dry period and 2.9 g P/kg DM during early lactation increased plasma Ca levels when compared with greater dietary P levels. These low P diets may be instrumental in preventing hypocalcemia in periparturient cows and do not compromise DMI and milk production. Current results suggest that P requirements in dairy cows during dry period and early lactation can be fine-tuned towards lower values than recommended by both the NRC (2001) and the Dutch Central Bureau for Livestock Feeding (CVB, 2012). Caution, however, is warranted to extrapolate current findings to entire lactations because long term effects of feeding low P diets containing 2.9 g/kg DM on production and health needs further investigation.